The IRS said I have an error in my refundable child tax credit and I don't know why

How old are your children?  To be a qualifying child for the 2021 tax year, your dependent generally must:

• Be under age 18 at the end of the year

 For tax year 2021, a qualifying child is an individual who did not turn 18 before January 1, 2022, and who satisfies the following conditions:

1. The individual is the taxpayer’s son, daughter, stepchild, eligible foster child, brother, sister, stepbrother, stepsister, half-brother, or half-sister, or a descendant of any of them (for example, a grandchild, niece, or nephew).
2. The individual did not provide more than one-half of his or her own support during 2021.
3. The individual lived with the taxpayer for more than one-half of tax year 2021. For exceptions to this requirement, see Residency Test in IRS Publication 501, Dependents, Standard Deduction, and Filing Information.
4. The individual is properly claimed as the taxpayer’s dependent. For more information about how to properly claim an individual as a dependent, see IRS Publication 501, Dependents, Standard Deduction, and Filing Information
5. The individual does not file a joint return with the individual’s spouse for tax year 2021 or files it only to claim a refund of withheld income tax or estimated tax paid.
6. The individual was a U.S. citizen, U.S. national, or U.S. resident alien. For more information on this condition, see IRS Publication 519, U.S. Tax Guide for Alien

https://www.irs.gov/credits-deductions/tax-year-2021-filing-season-2022-child-tax-credit-frequently-asked-questions-topic-b-eligibility-rules-for-claiming-the-2021-child-tax-credit-on-a-2021-tax-return

@Tyson509

there may be something more fundamental incorrect with your tax return as most people were eligible for up to $3600 per child

Did you earn less than $150,000 (joint) and did the children live with your for at least 6 months of 2021? if yes to both, then your tax credit should have been a lot larger.  Can you confirm whether you can answer 'yes' to both, in which case there is an error in how you entered critical information into TT.

but if you can't answer 'yes' to the 2 items above, it's odd, because everything to calculate the $4000 credit is automated by TT.  Look at how your two children are listed on the tax return.  there are two little check boxes WAY over to the right of their names - is the 'qualifiying child' box checked for both of them? 

Also, is the notice for 2020 or 2021 tax year?